The present invention relates to an improvement suitable for use with an encapsulated measuring system for measuring the relative position of two objects, of the type which comprises a housing, a measuring scale disposed within the housing, and a scanning unit mounted to scan the scale.
Such measuring systems are used to measure relative movement, as for example the relative movement between two machine parts. These two machine parts can be formed by the bed and the slide piece of a processing machine, for example. Typically, both the measuring scale and the scanning unit are enclosed within a housing, such as a hollow profile for example, in order to protect sensitive components of the measuring system.
In devices of this class of precision, the selection of the various component materials is of great significance, since materials having differing coefficients of thermal expansion typically must be combined. For a wide variety of reasons, it is only in the rarest of cases that materials of the same or similar coefficients of thermal expansion can be used. In many applications cast iron (which is used for machine parts) must be combined with aluminum (which is used for the hollow profile housing) along with glass or steel for the measuring scale. Furthermore, the work piece to be processed often may be made of steel.
It is a known practice of the prior art to mount the measuring scale to the housing in a manner such that the measuring scale is longitudinally shiftable with respect to the housing. For example, a layer of a highly elastic adhesive can be used to mount the scale to the housing. When this prior art approach is used, dimensional changes of the housing related to thermal expansion or bending have substantially no influence on the measuring scale. See, for example, the disclosure in German Pat. No. 11 76 382.
In addition, it is known from U.S. Pat. No. 3,816,002 to provide a measuring system in which the housing of the measuring system is fastened to the slide piece of a machine tool. The scale is fixed at one end inside the housing, while the other end of the scale is mounted by means of a tensioning device within the housing. This tensioning device serves to compensate by means of a spring for temperature dependent length changes of the housing. When mounted in this way, the scale remains substantially unaffected by temperature-induced length changes of the housing. The length of the scale, however, will change in correspondence with its own coefficient of thermal expansion. Such dimensional changes of the measuring scale can lead to measuring errors. It is typically necessary, therefore, to take into account measurement errors related to temperature variations in using the measuring system of this patent.
In such measuring systems, if the materials which make up the work pieces are changed, error compensation calculations have to be revised. The magnitudes of the differing coefficients of thermal expansion must be re-evaluated with regard to their effect on each new combination of materials.
German DE-OS No. 28 30 980 describes an arrangement for the correction of setting errors which occur in a measuring system as a result of thermally induced dimensional changes of machine parts. According to this disclosure, a first measuring interval (in the form of a drill rod) is firmly clamped in the installation plane of the measuring system on the machine part. Thermally induced length changes of this first measuring interval or drill rod result in a corresponding dimensional change of the processing plane of the tool relative to the installation plane of the measuring system. A parallel second measuring interval is rigidly joined with the first measuring interval, and this second measuring interval is positioned in the processing plane of the tool. The first measuring interval is characterized by a greater coefficient of thermal expansion than the second measuring interval such that if the measuring intervals are of equal length under normal conditions, temperature variations cause length differences between the two measuring intervals. This length difference is applied as a correction value to the measuring system for the detection of the change of position of the processing plane of the tool relative to the installation plane of the measuring system. This measuring arrangement is relatively expensive to implement, and requires that the second measuring interval be fastened to the first measuring interval in the processing plane of the tool. This is not possible in all measuring machines and the utility of this correction arrangement is therefore restricted to certain applications.
German DE-OS No. 25 58 625 discloses a machine tool which includes a scale for the correction of thermally induced spindle displacement. This scale is made up of an arrangement of several bars which are oriented parallel to one another. At least one of these bars is of steel, is fastened at one end to the machine, and is connected at its free end with at least one parallel bar of aluminum. The free end of this aluminum bar is joined in the vicinity of the fastening point with another bar of steel, on the free end of which is arranged a measuring sensor which interacts with a fixed point on the machine housing. This correction arrangement serves to establish a constant measuring interval, the length of which is substantially independent of thermally induced variations. This constant measuring interval is oriented parallel to a machine part, the thermal expansion of which is to be compensated. The measuring values determined by means of this constant measuring interval for the spindle displacement are applied to the drive control of the machine part which includes the spindle.
German Pat. No. 24 50 322 and German Pat. No. 31 06 701 disclose arrangements for the compensation of thermal length changes of machine parts. In these arrangements an expension bar with a greater coefficient of thermal expansion than that of the machine part is fastened with one end to the machine part and the other end to a scale which is mounted to be slidable in the measuring direction with respect to the machine part. The material and the length of the expansion bar are dimensioned in such a way that when the processing plane of the tool is shifted with respect to the scanning plane of the measuring system as a consequence of thermally induced length changes of the machine part, the scale undergoes a like direct displacement of the same amount in order to provide temperature compensation.